With an increasing demand for multimedia, large volume data such as digital still picture or dynamic picture are more and more dealt with. Such large volume data are usually stored in recording medium of large storage capacity, and the data as stored therein are randomly accessed to be reproduced as necessary. For such recording media, optical disks have been used for their superior characteristics as to random access approach, and high density recording over magnetic recording media like floppy disks, etc.
Among the foregoing optical disks, particularly for those capable of recording and reproducing information, magneto-optical media and phase change media are generally used in practical applications. A magneto-optical recording medium is structured such that a recording layer made of a ferromagnetic film of perpendicular magnetization is formed on a substrate. For such magneto-optical recording medium, recording of information is performed by focusing a light beam on the recording layer from an optical head while applying thereto a magnetic field from a magnetic head, so as to form magnetic domains corresponding to information to be recorded on the recording layer. On the other hand, reproducing of information is performed from the magneto-optical recording medium by detecting a change in Kerr rotation angle that varies depending on a direction of a magnetic domain formed in the recording layer.
On the other hand, for the phase change medium, recording of information is performed by focusing a light beam on the recording layer, and selectively forming a crystalline portion and an amorphous portion using the resulting heat from a light beam spot. On the other hand, reproduction of information from the phase change medium is performed based on a difference in amount of reflected light between the crystalline portion and the amorphous portion.
For both of the foregoing magneto-optical medium and phase change medium, when recording and reproducing, a light beam spot is subjected to a tracking control so as to follow exactly a track using protrusions and recessions called lands or grooves formed on the surface of the substrate as a tracking guide. In order to realize the optical disk of still larger storage capacity, the land and groove recording system wherein information are recorded on both lands and grooves has been adopted in many practical applications. For this land and groove recording system, the lands and the grooves are formed in virtually the same width.
In order to make a light beam spot exactly follow a track, a phenomenon in which a diffraction pattern of reflected light from an optical disk varies depending on the relative position between a light beam and a track is utilized. Known methods of utilizing the foregoing phenomenon include: a push-pull method for generating a tracking error signal from one beam, or three beam method for generating a tracking error signal from three beams, and a differential push-pull (DPP) method.
Here, a beam spot diameter is proportional to the wavelength of the light beam. Therefore, a smaller beam spot diameter can be obtained by reducing the wavelength of the light beam emitted from a light source. Specifically, developments have been made to use a blue color semiconductor later beam having a wavelength within a range of from 390 nm to 430 nm in practical applications. As described, attempts have been made to realize optical disks of still higher recording density by reducing the wavelength of a light beam emitted from the light source.
For optical disks adopting magneto-optical recording media, an optical disk for recording thereon and reproducing therefrom information by projecting a light beam having a wavelength of 655 nm through an objective lens having a numerical aperture of 0.65 to be focused thereon to form a beam spot having a diameter of around 0.9 μm has been developed. This optical disk includes a substrate having a thickness within a range of from 0.5 mm to 0.6 mm whereon lands and grooves are formed as recording tracks with a track pitch (groove width) of 0.535 μm. Hereinafter, this optical disk is referred to as a low density optical disk. In this low density recording optical disk, the groove depth is set so as to ensure an amplitude of a tracking error signal of not less than a predetermined level.
On the other hand, an optical disk which permits recording and reproducing at still higher density using a light beam having a shorter wavelength has been developed. This optical disk, for example, includes a substrate having a thickness within a range of from 0.5 mm to 0.6 mm whereon lands and grooves are formed as recording tracks with a track pitch (groove width) of 0.33 μm. For example, the optical disk for recording thereon and reproducing therefrom information by projecting a light beam having a wavelength of 410 nm through an objective lens having a numerical aperture of 0.65 to be focused thereon to form a beam spot having a diameter of around 0.5 μm has been proposed. Hereinafter, this optical disk is referred to as a high density optical disk. In this high density recording optical disk, the groove depth is set so as to ensure an amplitude of a tracking error signal of not less than a predetermined level.
For optical pickup devices adopting a light source for emitting a light beam of a short wavelength, development of those which permit recording and reproduction of information with respect to not only high density optical disks but also low density optical disks is desired, and particularly the development of those which permits at least reproduction of information from the low density optical disks is desired for sake of user's convenience. However, a difference in wavelength of light beams adopted causes a difference in optimum groove depth between the high density optical disk and the low density optical disk, and therefore it is expected to be difficult to perform desirable tracking servo for both high density optical disks and low density optical disks.
For a photodetector of the optical pickup device, an SiPIN photodiode is generally used However, the SiPIN photodiode is known to have such characteristic that a receiving light sensitivity varies depending on a wavelength of an incident light (the receiving light sensitivity corresponds to the efficiency for converting the light incident on the light receiving face into current).
For example, the SiPIN photodiode S6795 available from Hamamatsu Photonics Co., Ltd., according to the spectral sensitivity described in its explanation, has a peak receiving light sensitivity for a wavelength of 800 nm, and shows a significantly lower receiving light sensitivity for a blue beam in a wavelength band of 400 nm. The SiPIN shows receiving light sensitivities of 0.48 A/W, 0.22 A/W, and 0.2 A/W for the wavelengths of 655 nm, 410 nm, and 400 nm respectively. Therefore, assumed the receiving light sensitivity for the wavelength of 655 nm be 100 percent, then, the respective receiving light sensitivities for the wavelengths of 410 nm and 400 nm would be only 46 percent and 42 percent respectively.
In view of the forgoing, recently developments have been made to realize an SiPIN photodiode which offers an improved receiving light sensitivity in a short wavelength band. For example, S5973-02 available from Hamamatsu Photonics Co., Ltd., according to the spectral sensitivity described in its explanation, shows a peak receiving light sensitivity for a wavelength of 760 nm, and receiving light sensitivities of 0.44 A/W, 0.32 A/W, and 0.30 A/W for the wavelengths of 655 nm, 410 nm, and 400 nm respectively. Therefore, assumed the sensitivity for the wavelength of 655 nm be 100 percent, then, the respective sensitivities for the wavelengths of 410 nm and 400 nm would be 73 percent and 68 percent respectively.
However, even with the foregoing SiPIN photodiode which offers an improved receiving light sensitivity in the short wavelength band, due to variations in receiving light sensitivity with changes in wavelength of the incident light beam, it is still likely that its receiving light sensitivity drops particularly for the wavelength band of 400 nm. Therefore, even when adopting an optical recording medium which offers signals in the same light amount for the two different wavelengths, the output level of the signal from the photodetector drops, which in turn reduces the SN ratio of the tracking error signal, resulting in insufficient tracking servo.
Moreover, for the optical recording medium from which information are reproducible using light beams of two different wavelengths, the light beam of a shorter wavelength shows greater variations in signal level with changes in groove depth. Therefore, in the case of setting the groove depth so as to ensure a signal of a predetermined level for both of the light beams of two different wavelengths, a drop in signal level with changes in groove depth due to variations in the manufacturing processes becomes more obvious for the light beam of a shorter wavelength.
Furthermore, in the case of reproducing information from the low density optical disk using an optical pickup device for high density optical disk, provided with a light source for emitting a light beam having a short wavelength, another problem arises. That is, since a ratio of a spot diameter to a track pitch becomes too small, an area in which three beams of 0th order diffracted light beam and ± first order diffracted light beams interfere each other is formed at a central portion of a diffraction pattern of a light beam reflected from the optical recording medium due to guide tracks formed thereon. As a result, the waveform of a tracking error signal is distorted, and an error in counting tracks is liable to occur.